1. Field of the Invention
This invention relates to stitching systems utilizing a bobbin case assembly from which a stored supply of thread is drawn and, more particularly, to a bobbin case assembly having an associated thread tensioning element against which the thread paying off of the supply acts to produce controlled resistance to the payout of thread from the supply.
2. Background Art
In sewing/stitching operations, and particularly in embroidery operations, the tension of two source components forming the lockstitch needle thread and bobbin thread must balance to achieve a high quality stitch. If the tension in the needle thread is significantly greater than the bobbin thread tension, the bobbin thread can be pulled from through the underside of the fabric and show at the top side of the fabric being sewn. This condition can cause puckering of the fabric or disfigured sewing to occur. If the needle thread tension is significantly less than the bobbin thread tension, loops can form on either side of the fabric and the stitching formation can appear loose or distortedly large.
A primary job of a sewing equipment operator is to keep bobbin and needle thread tensions as close as possible to balanced. The method of balancing thread tension has historically been carried out by having the sewing operator observe the pattern after stitches are laid down. Good sewing operators constantly adjust the tension of both needle and bobbin threads to maintain the proper balance. Less skilled operators may not consistently maintain this balance as a result of which poor quality stitch formation may result.
The assignee herein is the owner of U.S. Pat. No. 6,152,057, which is directed to a bobbin case assembly with an associated tensioning element having a circumferential surface about which thread is wrapped to controllably increase thread draw tension. During setup, the sewing equipment operator can control the degree of wrapping of the thread around the tensioning element to thereby select the desired thread draw tension associated with that bobbin case assembly. This potentially obviates complex and time consuming adjustment procedures used in conventional sewing systems, which may incorporate a large number of sewing “heads”. While the system disclosed in U.S. Pat. No. 6,152,057 represents a tremendous contribution to the industry, there are some inherent limitations associated therewith.
First of all, in the event that a significant increase in draw tension is required, multiple wraps of the thread around the tensioning element may be required. This results in a spiral arrangement of the thread around the tensioning element. The spiral pattern of the wrapped thread may shift during operation relative to the tensioning element, which may result in an appreciable draw tension variation.
The industry continues to seek out ways to predictably select draw tensions, maintainable at or close to a desired value, without complicated setup procedures or excessive adjustment as the system is monitored both at start-up and during use.